It is known in the art relating to press forming of a sheet metal blank into a stamping, in particular stretch forming processes, to provide a lockbead around edges of the blank to hold the blank edges against inward motion during forming of the blank in a press. A conventional lockbead comprises a trapezoidal or nearly rectangular protrusion of the blank material upward or downward from the associated flange or edge portion of the blank.
The bead is usually formed in the press by engagement of the blank by one of upper and lower dies in the press and an associated binder (also called a cushion ring or blank holder). These members have opposed blank holding surfaces for engaging opposite sides of the blank adjacent its edges or periphery in what may be called the outer flange area of the blank. One of the opposing die and binder members is provided with a protruding bead of rectangular, square or trapezoidal cross section lying generally parallel to the blank edge. The other of the members has formed therein a mating bead recess or slot into which the protruding bead of the first member extends when the blank holding surfaces of the members are forced against opposite sides of the blank. This forms a nearly square, rectangular or trapezoidal bead in the blank running along its edges and trapped in the mating beads of the associated die and binder members.
A conventional bead, formed by a square protrusion forcing the blank into a parallel sided slot with small clearance, forms a nearly square sided bead in the blank having four sequential bends in the bead of nearly 90 degrees each, but slightly less because of the clearance required for bead forming. In stretch forming of sheet metal blanks, for example, the bead is usually capable of restraining the blank edges against inward motion if the angles of the four bends total 340 degrees or more (for example 85 degrees for each bend). This works well for horizontally disposed blank edge flanges but is less or not at all satisfactory where the blank edges are disposed at an angle, such as 45 degrees from horizontal. In such a case, the requirement that the slot edges remain vertical for entry of the protruding bead during a vertical motion of the press causes the bends in the bead to be substantially less than 90 degrees. The resulting sum of the bends then falls to significantly less than 340 degrees and sometimes less than 300 degrees, resulting in inadequate restraint of the flanges, or blank edges, against slipping, or inward motion during forming.